Recent Advances in the Prediction of the Thermal Properties of Syntactic Metallic Hollow Sphere Structures

Abstract

Metallic hollow sphere structures (MHSS) are a new group of cellular metals characterised by easily reproducible geometry and therefore consistent mechanical and physical properties. [1] Well-known advantages of cellular metals are their excellent ability for energy adsorption, [2,3] good damping behaviour, [4,5] sound absorption, [6] excellent heat insulation [7,8] and a high specific stiffness. [9,10] The combination of these properties opens a wide field of potential multi-functional applications.The high thermal insulation capability of hollow sphere structure has been addressed in a US Patent [11] by Schneider and coworkers. Baumeister and colleagues [12] investigated the linear thermal expansion coefficient of corundum based hollow sphere composites (HSC) using thermomechanical analysis. They found that the thermal behaviour of HSC is mainly governed by the epoxy resin. Lu and Kou [13] conducted a comprehensive numerical and experimental study based on unit cells of homogeneous hollow sphere structures. However, only automatically generated finite element meshes were used. This approach does not allow the consideration of different material combinations.A new powder metallurgy based manufacturing process enables the production of metallic hollow spheres of defined geometry. [14] This technology brings a significant reduction in costs in comparison to earlier applied galvanic methods and all materials suitable for sintering can be applied. EPS (expanded polystyrene spheres) are coated with a metal powder -binder suspension by turbulence coating. The green spheres produced can either be sintered separately to manufacture single hollow spheres (cf. Fig. 2)or be pre-compacted and sintered in bulk thus creating sintering necks between adjacent spheres. [15] Depending on the parameters of the sintering process, the micro-porosity of the sintered cell wall can be adjusted. In a subsequent de-bindering process, the EPS spheres are pyrolised. The increase of the carbon content of the sintered metal by the diffusion of the incinerated binder and polymer causes degradation of mechanical properties and corrosion resistance. Special reducing processes are required to reduce this effect. [16] Various joining technologies such as sintering, soldering and adhering can be used to assemble the single hollow spheres to interdependent structures. [17,18] Adhering is the most economic way of joining and therefore is attractive for a wide range of potential applications. Another important advantage is the possible utilisation of the physical behaviour and morphology of the adhesive layer as a further design parameter for the optimisation of the structure's properties for specific applications. Finite Element MethodThe numerical investigation of MHSS presumes the knowledge of its geometric dimensions. Therefore, measurements on experimental samples (cf. Fig. 1 a)) are performed. The averaged value of the outer sphere diameter R is 1.5 mm, the minimum distance a min between two neighbouring spheres is between 0.12 and 0.24 mm and ...

On the Anisotropy of Lotus‐Type Porous Copper

“…In comparison, finite element analysis only yields a single direction conductivity for each calculation. The LMC method has already been successfully applied towards the thermal analysis of hollow sphere structures,15, 16 random‐shaped cellular metals,17 and phase‐change materials 18. In LMC simulations, thermal conduction is modeled as the random walk diffusion of small energy packets 19.…”

Thermal analysis of aluminium foam based on micro‐computed tomography

“…Furthermore, the study revealed that the relative density influences the thermal conductivity more than the different polyhedron configurations of the structure. Fiedler et al [17] investigated the effective thermal conductivity of homogeneous syntactic metallic hollow sphere structures (MHSS) and heterogeneous adhesively embedded MHSS based on a periodic unit cell structure. In particular, different hollow sphere topologies (i. e. primitive cubic, body centred cubic and face centred cubic) and their influences on the free volume and effective thermal conductivity were examined.…”

On the thermal properties of expanded perlite – Metallic syntactic foam

“…In [15,16] the effective thermal conductivity of syntactic steel foams containing hollow spheres was predicted using numerical analysis. Conductivities were found to decrease from 35 W/m K to 20 W/m K with increasing filler particle fraction.…”